1. Field of the Invention
This invention relates to dissipating heat from a printed wig board using a heat pipe.
2. Description of the Related Art
One common process for definition of signal traces on printed wiring boards is known as a subtractive process. In this process a panel consisting of a copper clad dielectric material is coated with a photoimagable polymer (photoresist). A phototool containing a negative image (clear traces with a black background) of the signal trace layer to be formed is placed over the photoresist coated panel and ultraviolet (UV) light is used to expose and crosslink the photoresist through the clear areas in the phototool, which correspond to the subtractive signal image. Unexposed areas of the photoresist are then removed in a developer solution. The traces are then defined in the copper by exposing the panel to a copper etching solution, where the photoresist acts to protect the copper beneath it while the remaining copper is removed. The photoresist is then removed to complete the process.
A multi-layer printed wiring board typically includes several non-conductive layers of epoxy (a dielectric), several plated-on layers on each side of the dielectric, with spacer layers ("spacers") in between. Traces are etched in each layer to make the various interconnects. One of the many layers of the multi-layer printed wiring board is typically for power while another layer is for ground. The other (non-power and ground) layers are typically thinner and have narrower traces than the power layer or the ground layer.
Increases in semiconductor chip input/output (I/O) in high-performance computers and other systems, and the shrinking size of consumer and automotive electronics are driving the increased wiring density of integrated circuits defined in single and multi-chip packages and printed wiring boards. See Microelectronics Packaging Handbook, 2nd edition, pp. II-63, II-64 and II-117 (1997) by Tummala, Rymaszewski, and Klopfenstein.
To achieve increased wiring density, the signal traces on multi-layer printed wiring boards, and the spaces between the signal traces, are becoming narrower. In such high-density high I/O printed wiring boards, more pin-outs project from each integrated circuit and from all of the integrated circuits than in past practice. On a standard high-density high I/O printed wiring board there can be as many as one hundred integrated circuits, each with fifty to one hundred (or more) leads or pin-outs. As trace widths become narrower, the power density on both the integrated circuits and the printed wiring boards is increasing.
Heat pipes are sealed hollow tubes of copper filled with alcohol or other fluid at low vapor pressure, which gives them the ability to transfer significant amounts of heat along their length with minimal temperature differential. Heat pipes are attached to individual integrated circuits populated on a printed wiring board to dissipate heat from the integrated circuits. Heat pipes are used, for example, in laptop computers.
The position, size, shape and location of the integrated circuits on the printed wiring board are determined before a heat pipe can be permanently attached to an individual integrated circuit. The printed wiring board and the surrounding electronics are often custom designed to accommodate the rigid physical structure of the heat pipe. Access to the printed wiring board and other components in case of repair is often difficult. As such, there continues to be a need for a heat pipe arrangement which eliminates or reduces these concerns.